1. Field of the Invention
This invention relates generally to semiconductor fabrication and, more particularly, to injection of liquid silicon precursors into process chambers.
2. Description of the Related Art
Silicon-containing materials (e.g., pure silicon, silicon germanium, silicon germanium carbon, silicon carbon alloys, silicon carbide, etc.) are widely used in the microelectronic devices manufactured today. Commonly, these silicon-containing materials are part of silicon-containing layers that form various parts of the microelectronic devices. For a variety of performance and efficiency-related reasons, these devices are continually being made smaller. As the dimensions of these microelectronic devices decrease, however, the physical characteristics of deposited layers, including uniformity in thickness, composition, and coverage, are becoming even more strictly limited. Moreover, the transitioning of the semiconductor industry from wafers that are 200 millimeters (mm) in diameter to wafers that are 300 mm in diameter, and even larger wafers in the future, is further making the achievement of these stringent requirements for thickness, composition, and coverage uniformity more difficult.
Silicon-containing films are typically deposited using silane (SiH4) as a silicon precursor. Deposition of very thin films with uniform elemental concentrations represents a serious challenge for vapor deposition processes relying on conventional silicon precursors, however. Typical furnace-based deposition processes that utilize silane are generally unable to deposit continuous, smooth and homogeneous films having a thickness of 100 Å or less. Similarly, typical single wafer thermal CVD processes also suffer from an inability to deposit smooth, homogeneous thin film materials with a thickness of 150 Å or less.
Higher silanes such as disilane and trisilane are sometimes mentioned as silicon precursor alternatives to silane, but in most cases only silane was investigated. Disilane (Si2H6) is known to be less stable than silane, and in deposition experiments employing disilane it was reported that disilane gives poor step coverage and that the deposition reaction is too violent to be controlled within the temperature range of 400° to 600° C. See, e.g., U.S. Pat. No. 5,227,329. It is also known that trisilane (Si3H8) is even less thermally stable than silane.
Dischlorosilane (DCS) and trichlorosilane (TCS) have also been used as silicon precursors. These precursors are typically supplied in vapor form by use of bubblers, which are discussed further below.
The challenges related to meeting ever more stringent requirements for film uniformity are further exacerbated when forming compound silicon-containing films (e.g., silicon germanium, silicon germanium carbon, silicon carbon alloys, silicon carbide, etc.). Likewise, silicon-containing films can contain dopants and regulating the level of these dopants can also pose challenges.
In particular, many doped silicon-containing films have electronic band gap energies that are a function of their specific elemental composition. By incorporating dopant elements, primarily derived from the Group III and Group V families of elements, these semiconductors can be transformed into p-type (electron deficient) and n-type (electron rich) semiconductors. These doped films are the building blocks for a number of microcircuit devices, including transistors. The level and uniformity of doping in these layers influence the electronic properties of the layers, which in turn can influence the properties of the devices incorporating that layer.
Many dopants, e.g., halides, and precursors for compound silicon-containing films come from liquid sources, i.e., precursors that are liquids at room temperature and atmospheric pressure (“standard conditions”). In contrast to gaseous precursors such as silane, which can be directly flowed into a process chamber, a bubbler is typically used to deliver liquid precursors to a process chamber. Typically, a carrier gas such as nitrogen is bubbled through a heated liquid to pick up precursor molecules in vapor form and carry them into the processing chamber. Undesirably, the flow of the liquid precursor through a bubbler is indirectly controlled via control of the flow of carrier gas bubbled through the liquid precursor. Because the flow of liquid precursor is indirect and dependent on the vapor pressure of the precursor, metering the level of precursor entering a process chamber can be imprecise. Moreover, in cases where there is a mixture of liquid precursors, the amount of precursor taken up with the carrier gas is dependent on the vapor pressure of each of these precursors, further adding to imprecision in metering. This imprecision in the ability to meter dopants or other constituent parts of compound silicon-containing films further contributes to uniformity problems with deposited films, especially as device dimensions shrink and wafers grow larger.
In addition, bubblers have problems delivering materials having very low vapor pressures. For example, in the space between the bubbler and the processing chamber, these materials can condense if they are allowed to cool or may decompose if they are continually heated near the temperatures required for vaporization.
Consequently, there is a need for systems and methods of effectively delivering precursors to a process chamber to form silicon-containing layers that are highly uniform and conformal.